1. Technical Field
The present invention relates to a polishing composition which is employed for precise polish-finishing of metal, plastic, or glass, particularly employed for precise finishing of the surface of an aluminum magnetic disk (hereinafter the disk will be referred to as a xe2x80x9chard diskxe2x80x9d) which is installed in a hard disk drive of a computer.
2. Description of the Related Art
In recent years, as high-performance computers have been developed and computers have been downsized, there has been demand for high-quality mirror-surface finishing in hard disks without surface defects, in accordance with an increase in recording density of the disks. In order to meet such demand for surface finishing, there have been attained a variety of technical developments on polishing compositions, polishing pads, polishing machines, and polishing techniques.
For example, Japanese Patent Application Laid-Open (kokai) No. 62-25187 discloses a polishing composition containing an inorganic salt, serving as a polishing accelerator, such as nickel nitrate or aluminum nitrate for increasing polishing rate. Japanese Patent Application Laid-Open (kokai) No. 2-84485 discloses a polishing composition which contains an organic acid such as gluconic acid or lactic acid, and a sodium salt thereof for increasing polishing rate, and which can provide a polished surface with reduced surface defects. Japanese Patent Application Laid-Open (kokai) No. 7-216345 discloses a polishing composition which contains an organic acid, a molybdic acid salt, and alumina sol so as to attain a high polishing rate and a polished surface with reduced surface defects. These polishing compositions have been developed in order to maintain high rate of polishing a hard disk, reduce surface roughness and surface defects, and increase recording density.
Meanwhile, Japanese Patent Application Laid-Open (kokai) Nos. 5-2747 and 5-89459 disclose methods for reducing dub-off at the circumferential end of a hard disk and for increasing recording area, in order to increase recording capacity per hard disk. However, these publications do not disclose a polishing composition, although they disclose conditions for polishing. Japanese Patent Application Laid-Open (kokai) No. 1-263186 discloses a polishing composition containing triethanolamine carboxylic acid, triethanol hydrochloride, and aluminum stearate for reducing the amount of dub-off. However, since hard disks these days must meet very strict requirements concerning surface roughness, such a polishing composition cannot be directly applied to high-precision finishing of polished surfaces.
The polishing compositions disclosed in the above publications have been developed in order to enhance polishing rate, to reduce surface defects such as micro-pits and micro-protrusions as well as scratches on the surface of a hard disk for improvement of quality, and to reduce surface roughness for increasing recording density. Incidentally, there has been demand for increasing recording capacity in a hard disk of conventional size. In order to increase recording capacity of a hard disk, recording density per unit area in the disk must be increased, However, during polishing of a hard disk, the circumference of the disk is excessively polished to form a curve portion. Such an unavoidable curve portion is called xe2x80x9cdub-offxe2x80x9d or xe2x80x9croll-off,xe2x80x9d and a region containing dub-off-in a hard disk cannot be employed for recording. If the amount of dub-off can be reduced as much as possible, recording capacity per hard disk can be increased. Therefore, there has been demand for minimizing the amount of dub-off in a hard disk.
In view of the foregoing, an object of the present invention is to provide a polishing composition and method which can maintain polishing rate during polishing of the surface of a hard disk, maintain excellent properties and high quality of the surface of the disk, and provide a finished surface in which the amount of dub-off is considerably reduced as compared with that of a conventional level.
The present invention provides a polishing composition comprising water, a polishing material (particularly alumina), a polishing accelerator, and at least one of hydroxypropyl cellulose (hereinafter abbreviated as xe2x80x9cHPCxe2x80x9d) and hydroxyalkyl alkyl cellulose (hereinafter abbreviated as xe2x80x9cHRRCxe2x80x9d). The polishing composition is mainly employed for polishing the surface of a hard disk which is installed in a computer, but is not limited thereto. Particularly, the composition can provide a finished surface in which the amount of dub-off is considerably reduced as compared with that of a conventional level, while maintaining high polishing rate, high surface quality, and excellent surface roughness. The present invention also provides a method for polishing a workpiece using such a polishing composition.